XXH INTRODUCTION. 



stages of increase may be pretty readily dis- 

 covered in the majority of shells, they being, for 

 the most part, marked on their convexity with 

 little eminences, parallel to each other, of different 

 degrees of strength, giving the surface a fibrous ap- 

 pearance. These eminences, which are called stria, 

 follow the contour of bivalve shells, and run in a spiral 

 direction in the univalves. It will be observed, upon 

 examining the different species, that they, for the 

 most part, have some of these eminences particu- 

 larly distinct, and distant from each other. These 

 denote the different times when the shell was inter- 

 rupted in its growth, and bear some analogy to the 

 nodes observable in the shooting of trees. 



Another circumstance, which distinctly marks the 

 place where the growth has recommenced, is a 

 striking change of colour in the bands which di- 

 versify some of the univalve shells. In these parts 

 the bands are generally much lighter, and sometimes 

 so faint as scarcely to be perceived, till they have 

 proceeded further on the spire. This is very evi- 

 dent in the exotic snail (Helix pomatia), where a 

 strong line of separation may be perceived from 

 the umbilicus across the shell. It will not be diffi- 

 cult to account for this change, if we consider that, 

 during the inactive state of the animal, the co- 

 louring matter contained in the collar is exhausted 

 on the last formed portion of shell, and that it will 



